Well you know, I posted a work around on page 3, that allows you to get both steam achievements and xbox live achievements (and you end up on the leaderboards as well) that still works and will always work unless 343i changes how the checkpoint system works.

… This means that among other things, you can adjust bloom from 0% to 200%, enable a full-auto Magnum (so that holding the trigger fires the pistol repeatedly), and tune the Magnum's fire rate.
Currently, only multiplayer and Forge variants are supported; Firefight and Campaign variants are not.